Interestingly,

1) The number of grains is about 10^123. Number 123 sounds familiar.
2) Changing the size (radius) of the universe from 15 billion years to say 30 billion years does not change the size of the volume grain that much, which is about the size of an electron! Wikipedia says: "The classical electron radius is 2.8179 × 1015 m"
3) You can try to back calculate the size (radius) of the universe, given that the size of the volume grain is equal to the size of the electron.. It turns to be 27.2 billion years (which might be about twice as much as the size of the currently visible universe.)

Here are the back of the envelope calculations:

Radius of the universe, assume, = 15 billion light years = 4.73*10^17 light seconds
Speed of light = 3.00 * 10^8 m/s
Universe radius, r = 3.00*10^8 m/s * 4.73*10^17 s = 1.42*10^42 m
Area of the sphere of the universe, A = 4*pi*r^2 = 2.53*10^53 m^2
Volume of the sphere of the universe, V = 4/3*pi*r^3 = 1.20*10^79 m^3

Planck distance = 1.616*10^-35 m
Number of pixels (grains of the size of Plank distance) on a square meter = 3.83*10^69
Number of pixels (grains) on the sphere, Ns = 9.69*10^122

size of volume pixels (grains) in the volume of the sphere, rV = 1/ [ ( Ns / V ) ^ (1/3) ] = 2.13*10^-15m

(check: number of volume grains in the universe = number of grains in 1m^3 * volume of universe in meters = (1/2.13*10^-15)^3 * 1.20*10^73 m^3 = 9.69*10^122)
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